1. Field of the Invention
The present invention relates to an optical pickup apparatus of an optical disc apparatus which is installed in a personal computer and the like and which records or reproduces data by the use of light.
2. Description of the Related Art
An optical pickup apparatus of an optical disc apparatus irradiates an optical disc with a light beam, receives reflected light to read a signal, performs feedback control so as to minimize servo error signals read from a track on the optical disc, and makes the light beam follow the track on the optical disc.
Servo error signals include two types of error signals, one of which is of focusing error signal (abbreviated as FES from now on) and the other of which is of tracking error signal (abbreviated as TES from now on). An FES is a signal which indicates a deviation in a focusing direction with respect to an optical disc, and maintains predetermined conditions with respect to a space between an object lens and a data recording face of the optical disc and a relative position between the object lens and a pit line of the data recording face into given conditions. On the other hand, a TES is a signal which corresponds to a deviation of light collected on an optical disc from the center of a track, and based on this signal, a light beam is made to correctly follow the track.
In cases where there is a scratch on an optical disc, a servo error signal becomes too large owing to the scratch and a servo often deviates from a track followed at present, with the result that such a problem occurs that data becomes incomplete while a CD-R (compact disc-recordable) is read out.
Further, as a conventional optical pickup apparatus is disclosed “a light-receiving/emitting integral device and an optical pickup using the same” in Japanese Unexamined Patent Publication JP-A 2001-110085 (2001). In the light-receiving/emitting integral device and optical pickup using the same disclosed in the publication, a lightproof mask is provided for the purpose of preventing random light that a light beam emitted from a semiconductor laser device is reflected on a surface of an object lens toward an optical detector, and provision of the lightproof mask makes it possible to detect a stable signal.
Characteristics of light in cases where there is a scratch on a surface of an optical disc will be explained referring to FIG. 14. FIG. 14 is a view showing a condition where scattering light is caused in a direction orthogonal to a scratch 94 on a surface of an optical disc 8. As shown in FIG. 14, in cases where the scratch 94 exists on the surface of the optical disc 8, a light beam from a semiconductor laser diode is scattered by the scratch 94. On a linear scratch, light is easy to scatter in a direction orthogonal to the direction of the scratch, whereas hard to scatter in a direction parallel to the scratch, so that a radiation pattern of scattering light caused on a linear scratch has a nearly oval shape which is longer in a direction orthogonal to the direction of the scratch.
Of the scratches 94 on the surface of the optical disc 8, a scratch in the tangent direction of a track is easy to cause track servo deviation due to influences of the scratch because a time to pass the scratch becomes long as compared with a time to pass a scratch in the radial direction orthogonal to the track.
A method of detecting a TES necessary for a track servo in an optical pickup apparatus will be explained. A light beam from a semiconductor laser diode is divided into three beams by a grating, and one main beam and two sub beams are emitted to a track on an optical disc in nearly parallel while low angles are formed. That is to say, the beams are emitted in a manner that the sub beams are displaced by a half of a track pitch with respect to the main beam. FIG. 10 is a view showing a state where a main beam (M) and sub beams (S1, S2) are collected on an optical disc in an optical pickup apparatus.
FIG. 15 is a view showing a light-emitting device and an entering pattern of scattering light caused by a scratch of an optical disc in the prior art. As shown in FIG. 15, three divided beams are diffracted at a hologram pattern 4 which is divided into three domains HA+B, HC and HD, respectively, and divided into nine beams. The nine beams are entered into a photoreceptive domain for focusing error signal (hereinafter abbreviated as PDf) and a photoreceptive domain for tracking error signal (hereinafter abbreviated as PDt). The PDf is divided into two photoreceptive portions A, B, and the PDt is divided into six photoreceptive portions C, D, E, F, G, H. A main beam from the HA+B domain of the hologram pattern 4 enters the center of the photoreceptive portions A, B of the PDf, three beams from the HC domain enter into the photoreceptive portions E, C, G of the PDt, and three beams from the HD domain enter into the photoreceptive portions F, D, H of the PDt. When signals from the respective photoreceptive portions A to H are denoted by A to H, a TES can be found by an equation below:TES=C−D−{(E+G)−(F+H)}  (1)Here, the relation between the signals A to H and reflection signals of three points collected onto the optical disc becomes a relation shown in FIG. 10. Such a detection system of detecting imbalance of distribution of reflected light caused by displacement between a track and a light beam by using an optical detector to obtain a TES as described above is called differential push-pull, and this system is suitable for a recordable type (CD-R) and a rewritable type (CD-RW).
In cases where there is a scratch on the optical disc, M, S1, S2 are scattering light and therefore cannot be focused in a dot state onto the photoreceptive portions of the PDf and the PDt, so that such a radiation pattern of scattering light as shown in FIG. 15 is projected on the photoreceptive portions of the PDf and the PDt.
Of scattering light, M is set to have a large amount of light as compared with S1, S2 (in general, S1:M:S2=1:10:1), with the result that scattering light from M becomes a leading cause of making a TES abnormally large.
Scattering light 93 from M enters almost the center of the photoreceptive portions A, B of the PDf and the photoreceptive portions C, D of the PDt, scattering light from S1 enters the photoreceptive portions E, G of the PDt, and scattering light from S2 enters the photoreceptive portions F, H of the PDt. That is to say, the layout of the three beams on the optical disc is projected onto the photoreceptive portions E, C, G and F, D, H of the PDt as it is.
Further, in cases where there is a scratch in the tangent direction of the track on the optical disc, scattering light due to the scratch shows a radiation characteristic of expanding in a direction orthogonal to the direction of the scratch, that is, the radial direction of the track and a direction orthogonal to an alignment of the three beams.
Therefore, the scattering light 93 from M due to the scratch in the tangent direction of the track is projected as a pattern of expanding in a direction orthogonal to an alignment direction of the photoreceptive portions E, C, G or F, D, H on the PDt.
At this moment, scattering light of the three beams enters the photoreceptive portions A, B of the PDf and the photoreceptive portions C, D of the PDt. In cases where the scattering light 93 from M having a large light amount of the three beams enters the photoreceptive portion G or F of the PDt, a signal of G or F shows an abnormally large value in a TES of the equation (1). Therefore, a TES becomes abnormally large and a servo deviates from a followed track. In this specification, “a servo deviates from a followed track” may be expressed as “a track servo deviates.”